1. Field of the Inventions
The invention relates generally to demodulating a signal and determining its carrier frequency and particularly to using the same circuit to perform both tasks.
2. Background Information
In a typical consumer infrared (CIR) system, a digital signal is used to communicate between devices such as between an electronic product and a remote control (RC). This digital signal is usually of low rate such as 1 or 2 bits per millisecond. A high data rate is not required in these applications since the amount of information conveyed by a command is usually very small.
The actually encoding scheme can vary depending on the system. FIG. 1A illustrates a signal where a fixed symbol period is used. During each period, information can be expressed. This example is a portion of a message encoded with the Philips RC-5 protocol. Each symbol comprises two half period where the signal is high during one of the two half periods and whether the bit conveyed is a “1” or “0” depends on which half is high.
FIG. 1B illustrates a signal where a variable symbol period is used. The example is a portion of a message encoded with the Philips RC-6 protocol. As can be seen, the first symbol which is primarily used for timing is long, followed by several short symbols and an intermediate length symbol. While the specifics of each encoding scheme are not important to the understanding of this disclosure, it should be noted that in general signals from RCs are sequences of high and low signal with either fixed or variable symbol periods.
Due to the low data rate, ambient light sources could potentially interfere with the CIR signal. For example fluorescent lights flicker at 60 Hz and may produce light in the infrared region used by the CIR device. Additionally, the photodetectors used in the CIR receivers may not be tuned specifically to a narrow infrared frequency, inviting optical interference from a variety of sources. For this reason, CIR signals are used to modulate a carrier signal. Typically, the use of a carrier signal enables the receiver to filter out noise for example through the use of a notch filter.
FIG. 2 conceptually shows the modulated signal. As this is an example, it should not be taken that the actual number of pulses shown is a true relationship between the unmodulated signal and the modulated signal. FIG. 2 is a magnified view of the portion of the signal highlighted in FIG. 1B. Depending on the manufacturer carrier frequency can vary between 30 kHz and 65 kHz.
FIG. 3 illustrates an exemplary receiver circuit for a CIR receiver. An IR is received by photodetector 302 which can be implemented using a photodiode or other methods that are well known in the art. The signal is then amplified by amplifier 304 which is often a transimpedance amplifier. Not only does the amplifier boost the signal received by photodetector 302, but it is often used to convert the current to a voltage. Typical photodetectors produce a current proportional to the optical power seen, but most logic circuits use voltage to transmit signals. The amplified signal is then limited by limiter 306, which is often a limiting or saturating amplifier. The limiter 306 helps to insure a full logic level is obtained. The signal is then filtered using filter 306 which can be a band pass filter allowing essentially the carrier signal or range of potential carrier signals through. The signal is then demodulated by demodulator 310 and decoded by decoder 312. Decoder 312 can pass on the message or command received to an appropriate circuit for use. Often the decoder comprises an integrator and a comparator to extract the information.
In a typical receiver, the carrier frequency and the encoding methods are known. As a result, filter 306 and demodulator 310 can be tuned specifically to the carrier frequency and decoder 312 can extract the command or message sent by the RC. However, for a universal receiver, the carrier frequency and encoding methods are not precisely known. The receiver may know for instance that the carrier is one of many, but not which of the many. For a universal CIR receiver, there can also be a requirement that the carrier frequency be provided along with the command or message. To complicate the situation further, the determination of the carrier frequency can be required to be obtained simultaneously with the decoding of the command or message, that is, no time is allotted to carrier frequency determination. Accordingly, various needs exist in the industry to address the aforementioned deficiencies and inadequacies.